A suite of synthetic anorthite crystals with different states of Al-Si order, from
C1 with short-range order through incommensurately ordered to commensurately ordered with the thermodynamically stable
I1 ordering scheme, have been used for lattice parameter and high-temperature solution calorimeter measurements. The macroscopic order parameter,
Qod, describing the degree of order with respect to the
C1 ⇌
I1 transition, has been followed through its relationship with the spontaneous strain,
. To a first approximation, the excess enthalpy due to ordering is linear with
, giving a minimum value of −3.3 ± ~ 1 kcal/mol for the enthalpy of the exchange reaction (-Al-O-Al-) + (-Si-O-Si-) → 2(-Al-O-Si-). Both the strain and enthalpy data show that, for a limited range of
Qod, the kinetics of ordering can be described by an empirical rate law of the form
Values of
A = −2.1 × 10
−5,
Tc = 2215 °C, Δ
H* = 124 ± ~15 kcal/mol, and τ
0 exp(−Δ
S*/R) = 9 × 10
−21 s were obtained. This type of rate law can be understood in terms of solutions to the Ginzburg-Landau rate equation for systems in which the degree of order is inhomogeneous on a local scale.
Type b antiphase domains (APDs) in the more ordered samples have been characterized by transmission electron microscopy, allowing the relationship between excess enthalpy and APD size, δ, to be determined. Treating the boundaries between APDs as surfaces yields defect energies of 1.22 ± ~0.30 × 10−5, 0.94 ± ~0.20 × 10−5, and 0.67 ± ~0.20 × 10−5 cal·cm−2 for crystals annealed at 1400, 1300, and 1200 °C, respectively. The apparent decrease in these defect energies with decreasing temperature suggests that the antiphase boundaries are stabilized by Al-Si ordering on the basis of their local C1 symmetry. They might even become stable features of the crystals at some low temperature, with implications for the nature of periodic antiphase boundaries in the intermediate plagioclase structure.
